[clang] Patch series to reapply #118734 and substantially improve it (PR #120534)

[Chandler Carruth via cfe-commits]

chandlerc wrote:

> > > @dyung - I believe this PR may be a credible path to address the issues hit with your MSVC builders, would appreciate any help testing it in advance if possible.
> > 
> > 
> > Sure, I'll give it a try
> 
> You seem to still be working on it, can you tell me which commit I should try building/testing when you are done?

Sorry, was just tidying up things that I missed until the PR was uploaded. Only rebasing on top-of-tree and a clang-format issue, nothing that changes functionality. And I think all done now so you should be good-to-go with the top of this PR (I think [`2bcc4e5` (#120534)](https://github.com/llvm/llvm-project/pull/120534/commits/2bcc4e5f7043dab1ef673dd20b38009363db51db) is the link to that commit)

https://github.com/llvm/llvm-project/pull/120534